Many first-time water gardeners can watch water lilies grow and not realize how quickly they overgrow. Ken Landon, Tim Davis and Zac deGarmeaux provide a video on how-to correctly repot an overgrown and overcrowded lily installation. The video provides step-by-step details of what to look for, such as a growth point of foliage, flower buds and the crown.
Pondmegastore.com provides more detail below:
Rhizomes grow in straight lines so point the growth tip in a direction to grow across a pot. When it reaches that side of the pot. It is time to re-pot it again. Some hardy lilies grow rapidly others are slower. N. Wanvisa (as in video), N. mexicana are two fast types of growing lilies. Do not cover the crown of the plant with stones, common practice with new water gardeners.